Deallocate PTY's when they close.

This required a fair bit of plumbing. The CharacterDevice::close() virtual
will now be closed by ~FileDescriptor(), allowing device implementations to
do custom cleanup at that point.

One big problem remains: if the master PTY is closed before the slave PTY,
we go into crashy land.

Kernel/SlavePTY.h

@@ -22,6 +22,7 @@ private:
     // ^CharacterDevice
     virtual bool can_write(Process&) const override;
     virtual const char* class_name() const override { return "SlavePTY"; }
+    virtual void close() override;
 
     friend class MasterPTY;
     SlavePTY(MasterPTY&, unsigned index);